The present invention relates to a method for preparing catalysts, and in particular to a method for impregnating monolithic catalyst supports, such as honeycomb structures, which substantially reduces preferential loading and achieves unit selectivity.
Catalyst impregnation is one of the most crucial steps for preparing industrial catalysts, such as those useful in hydrotreating applications. Generally, high surface area oxide substrates or supports are loaded with one or more catalytically active materials, such as noble and/or base metals. Two widely-known impregnation techniques include pore volume impregnation (PVI), and excess solution impregnation (ESI).
PVI which is used with pellets or beads catalyst supports, involves the use of a catalyst solution of a volume equivalent to or slightly less than the total pore volume of the supports. The liquid solution penetrates and fills up the support pore volume, delivering a specific amount of metal into the pores. Following imbibation (i.e., complete filling of the pore volume with the available liquid solution) the pellet supports are dried to remove excess water, and achieve the desired metal concentration. Advantages of the PVI technique include easy determination of the catalyst volume solution required, and unit selectivities (i.e., the weight ratios of the catalytically active materials in the solution are equal to the weight ratios of the catalytically active materials in the substrate).
However, pore volume impregnation is not readily applicable to monolithic catalyst supports due to the complexity and rigidity of their shape. Simply absorbing the volume of solution equivalent to the volume of the support, generally results in very poorly distributed catalysts. Therefore, honeycomb supports are usually impregnated with a catalyst solution having a volume that exceeds the support's pore volume. This technique is known as excess solution impregnation or ESI. The honeycomb support is fully submerged in the catalyst solution over an extended period of time. The solution penetrates the pores of the support due to capillary pressure and fills up the available pore volume.
A disadvantage of the ESI technique, is the interaction that occurs between the surface of the support and the catalyst solution because the various catalytically active materials have different chemisorption rates or react at different rates and to different equilibrium levels. This leads to “preferential loading” of some metals over others, and unit selectivities cannot be achieved.
It is extremely important for any large scale impregnation process to have selectivities as close to unity as possible, in order to be commercially viable. If the selectivities are far from unity, the relative metal concentrations in the solution will differ before and after impregnation and the composition of the solution will change over time. This renders the catalyst solution unusable after a period leading to material loss and excessive cost. Further, to achieve the desired catalyst composition, the impregnation solution must be optimized independently for each process, which results in further costs and inefficient processing.
There is, thus, a need for an improved method to impregnate porous monolithic supports to reduce preferential loading and achieve unit selectivities, and provide a process that is cost-effective and efficient.